Most electric vehicles are based around designs used for internal-combustion-powered vehicles, with modifications as necessary to replace the internal combustion engine with an electric motor and to store the necessary batteries at a suitable location within the vehicle. This has the effect that the chassis of the original vehicle is inherited by the electric vehicle, with little adaptation to take account of the substantial weight of the batteries and little thought given as to how they might be removed and replaced in future. Generally, batteries gradually lose capacity as they are cyclically charged and discharged and do eventually need to be replaced.
As examples, in the BMW Mini ‘E’ vehicle, the electric motor is located in conventional location in the front engine bay and the batteries take the place of the rear seats and part of the rear loadspace, leading to a design that is able to accommodate only two people despite having a relatively large bodyshell. The Toyota Prius hybrid integrates the batteries into the vehicle more effectively, with the result that their replacement is regarded by some as not economically viable, implying a service life for the vehicle of only 150,000 km (approx 100,000 miles). Only a few vehicles have been designed ab initio around the needs of electrical propulsion, such as the Mercedes A-class which includes a double-floored chassis that provides space beneath the passenger cabin in which batteries can be placed.
This approach hampers the design of electric vehicles. The inherited chassis is usually of a pressed steel construction, which carries a substantial weight penalty. This, in turn, means that more powerful batteries are needed in order to propel the vehicle, increasing the weight of the vehicle still further; each cell is typically 6 kg, and many cells will be needed for a viable vehicle. However, a strong crash-resistant chassis or other form of restraint for the batteries is essential in order to restrain them during an impact. Often, the batteries are located behind the passengers and would therefore inflict serious injuries if unrestrained during a frontal collision. Such an enclosure or restraint is usually of steel or other similar construction, adding still further weight. Designing this in a way that allows for easy replacement of the batteries will add further complexity and weight.